1. Field of the Invention
The present invention relates to a process for the transmission and reception of personalized programs.
2. Discussion of the Background
The great increase in the number of transmission or broadcasting supports or carriers (satellites, cables, microwave) has led program operators to vastly increase offers of services for access control programs, which can be in the form of audiovisual, sound or data transmission programs.
The principle of access control to such services is based on the scrambling of a program on transmission and the descrambling of the message received under the control of an access title. The scrambling and descrambling systems are initialized by a data, which varies in a random manner and which is known as a control word. The informations describing the access criteria to the program and a protected form of the control word are included in control messages of the access titles and which accompany the scrambled program.
To have access to a scrambled program it is necessary for the conditional access device to be able to exploit one of the control messages of the access titles associated with the program. In the case of "personalized" services for which the program is constituted by a plurality of program elements (video sequences and/or sounds and/or data) addressed to the same number of different addressees or recipients, each program element must only be restored in uncoded form for the single recipient involved and the control messages for the access titles must make it possible to guarantee the protection of each of the program elements.
The present invention relates to a process for the transmission and reception of access title control messages enabling any recipient (but him alone) to receive the program elements sent to him within the scope of a personalized transmission service.
The applications of such a process are all personalized program transmission or broadcasting services multiplexed on the same support, e.g. vocal messaging, data transmission (telex, data downloading, fax, radio messaging such as operator or alpha paging), fixed or moving picture transmission (video transmission), etc.
Existing processes making it possible to protect access to a broadcast program consist of allocating to each program element an access title control message. This method is widely used in pay television services for which each program or program element relates to a large audience and therefore for which it is sufficient to associate one or more, typically three or four access control messages with the complete broadcast program. The information rate specific to the access control relative to a broadcast program is then very small (typically a few hundred bytes per second for controlling an information rate of several megabytes per second).
Each access title control message generally has four fields:
an identifier of the service key to be used, PA0 a field defining the access conditions to be satisfied to have the right to use such a service key, PA0 the cryptogram or cryptograms of one or both control words, PA0 a redundancy field, which can be usefully added in order that the security processor cannot be used outside the intended context. PA0 the programs are scrambled by a control word and access control messages are formed which in particular contain access criteria and a cryptogram of the control word, PA0 a check is made to establish whether the access criteria are fulfilled, the control word used for scrambling is reconstituted and the received programs are descrambled, PA0 the control word used for scrambling the program intended for the particular recipient is obtained by personalization, with the aid of the identifier of said recipient, of a single control word called the root control word and which applies to all the recipients and only a single access control message is transmitted for all the recipients, said message more particularly containing a cryptogram of the root control word, PA0 each recipient, with the aid of his identifier and the access control message, reconstitutes, on the basis of the root control word, his own personalized control word, which enables him and him alone to descramble the program intended for him.
When the security processor contains an appropriate access right, i.e. when it holds the service key indicated by the identifier and said key is provided with a status compatible with certain conditions indicated in the access criteria field, the security processor decrypts or deciphers the cryptogram or cryptograms for reconstituting the control word or words. The control word enables the terminal to descramble the program or program elements with which it is associated.
In the case of personalized broadcasting services, the extension of the preceding method, which would consist of associating the same number of access title control messages as there are program elements accessible in a distinct manner, would make it necessary to reduce in certain cases, the information resource by two, assuming that the content of the information to be transmitted is roughly the same size as the actual access title control message (case of radiomessaging displaying 40 characters). This considerable decrease in the remaining useful resource i.e. the information is contrary to the economic profitability sought for the broadcast service.